The indicator serves to display the present values of a first speed of rotation corresponding to the main rotor and a second speed of rotation corresponding to the free turbine of the engine, respectively written Nr and NTL by the person skilled in the art, while also being capable of predicting changes in the first speed of rotation during stages in which the engine is accelerating or decelerating, referred to as “transient” stages. Such an indicator is more specifically adapted to a helicopter.
In a rotorcraft, such as a helicopter, it is necessary to measure and display on the pilot's instrument panel, both the first speed of rotation, corresponding to the main rotor, and the second speed of rotation, corresponding to the free turbine(s) of the engine(s).
A first system is known for a single-engine rotorcraft, that displays the first and second speeds of rotation Nr and NTL on a display screen by means of respective first and second needles. During normal operation of the rotorcraft, the two needles are aligned, i.e. they are superposed. Loss of needle alignment indicates that there is a mechanical problem in the power transmission assembly of the engine, e.g. in the main transmission gearbox.
A variation of that system for use in a two-engine rotorcraft displays the first speed of rotation, corresponding to the main rotor, and also the second speeds of rotation, corresponding to the free turbines of each of the engines, which implies using three distinct needles.
As mentioned above, such commonplace systems enable the pilot to detect a possible mechanical problem situated in the power transmission unit.
In addition, when the first speed of rotation Nr crosses a limit threshold, whether a maximum or a minimum, an audible alarm is issued so that the pilot can change the maneuver being performed. This alarm is of great importance since the first speed of rotation Nr is vital for flight safety, too slow a speed leading to loss of lift, for example.
Although they are effective, those prior art systems turn out to be insufficient in preventing the first speed of rotation Nr, corresponding to the main rotor, dropping or increasing too quickly. Experience shows, e.g. when maneuvering too fast, that the first speed of rotation Nr can exceed its maximum limit threshold in a length of time that is shorter than the reaction time of the pilot, which can lead to a situation that is catastrophic.